Generally, a golf club comprises a shaft portion, a head portion, and a grip portion. That part of the golf club head portion which outlines or defines a hitting surface is called a golf club face. See, e.g., R. Maltby, "Golf Club Design, Fitting, Alteration & Repair" (4th Ed. 1995). Generally, a club face abuts or is adjacent to both a crown (or top portion) of the club head and a sole (or bottom portion) of the club head.
In hollow metal wood type club heads and cavity backed iron type club heads the golf club faces are preferably thin. Such golf club faces generally define two surfaces: a hitting (or front) surface and a back surface which is opposite the hitting surface.
When the face of a golf club head strikes a golf ball, large impact forces (e.g. up to 2000 pounds) are produced. These large impact forces load the club face. In the relatively thin faces of hollow metal wood type club heads and cavity backed iron type club heads these forces tend to produce large internal bending stresses. These internal bending stresses often cause catastrophic material cracking which causes the club head to be unusable.
Recent computational and experimental studies on hollow metal wood type club heads and cavity backed iron type club heads have shown that such catastrophic material cracking most often occurs in at least one of the following three face locations: (1) in the head face hitting surface at the ball strike center which is an area of large compressive bending stresses, particularly in the area of any score-lines; (2) on the back surface of the head face at the ball strike center which is an area of large tensile bending stresses; and (3) (a) at the portion of the intersection of the face and the crown which lies directly above the ball strike center which is an area of large vertical component of the bending stresses, and/or (b) the intersection of the face and the sole which lies directly below the ball strike center which also is an area of large vertical component of the bending stresses. The region between the face/crown intersection above the ball strike center and the face/sole intersection below the ball strike center may be called a ball strike zone.
It has also been found that the vertical stress distribution through the ball strike zone on the back side of the face comprises large compressive (i.e. negative) stresses in the face/sole intersection region which increase to zero toward the ball strike center region, reach a maximum tension (i.e. positive) value behind the ball strike center region, decrease through zero to large compressive (i.e. negative) stresses toward the face/crown intersection region. The vertical stress distribution through the ball strike zone on the front side (or hitting surface) of the face generally has the same, but opposite, components (i.e. large tension bending stresses at face/sole intersection which decrease to large compressive stresses at ball strike center and then increase to large tension bending stresses at face/crown intersection).
In designing golf club heads, the golf club face portion must be structurally adequate to withstand large repeated forces such as those associated with ball impact. Such structural adequacy may be achieved by increasing the face portion stiffness so that the bending stress levels are below the critical stress levels of the material used in the face. Typically, for metal club heads, the face portions are stiffened by uniformly increasing the thickness of the face portion and/or by adding one or more ribs (i.e. discrete attached posts or metal lines) to the back surface of the face.
Uniformly increasing the thickness of the face portion typically requires the addition of a large amount of material to adequately reduce the stress sufficient to prevent impact and/or fatigue cracking. However, the addition of such a large amount of material to a club face generally adversely affects the performance of a club incorporating such a face. The club performance is adversely affected by the overly heavy club head which has a mass center (i.e. center of gravity) which is too close to the club face thereby affecting optimum performance. In addition, the feel and sound of a club incorporating such a face is also adversely affected by the large number of vibrations transmitted through the club and by the acoustic response of the club.
Adding ribs to the back surface of a face to stiffen the face has the benefit of stiffening without adding a significant amount of weight to the face, but has the detrimental result of creating an irregular stiffness distribution on the face hitting surface. Examples of ribs which have been used in prior metal golf club head designs include, for example, vertical ribs, horizontal ribs, curved ribs, dendritic ribs, angled or skewed (i.e. V or X patterned) ribs, circular ribs, or a combination of more than one of these types. Such ribs are generally geometrically characterized as having a narrow width, any desired length, and a sufficient depth or thickness to locally increase the face stiffness and yet minimize the increase in face weight.
In addition, such ribs are typically shaped such that a sharp corner (or a curved corner with a small radii) is formed between a rib and the face back surface where the rib is attached. Such corners lead to cracking potential. Furthermore, the use of ribs which are positioned to run vertically along the face back surface cause the large bending stresses (which were described above) to travel to the face/sole and face/crown intersections thereby increasing cracking at those positions.
Additional problems experienced with the use of ribs on a face back surface are in the manufacture of such faces. Typically faces are formed using a casting process. It is more difficult to cast faces which include rib structures due to nonuniform material shrinkage which occurs during cool-down of such a casting. Such non-uniform cool-downs tend to cause inclusions, internal voids, and/or surface cracking in the cast materials, particularly along regions where ribs are positioned. Such non-uniform cool-downs also tend to cause face depressions and surface dimpling in the hitting surface opposite the regions where ribs are positioned.
Thus, there is a need for a new club face structure with increased structural integrity (and, thereby, reduced cracking and material failure) without adversely affecting club performance, look, feel, and sound.